[The] frontier metaphor has been basic to Heinlein's writing. Only eight of his … novels take place primarily on Earth, and four of them concern relations between humans and intelligent extraterrestrial beings, while a fifth concludes on the Moon. This outward spatial movement, coupled with a forward temporal movement, places Heinlein's characters in situations of extremity, facing the unknown and having to learn to understand it, in order just to survive. Whether they are in spaceships or on alien worlds, exploring or settling or righting wrongs—fighting off other species or learning to live with them, their situations parallel those of the American pioneers, for all that they are equipped with advanced technology, "scientific" thinking, and the benefits of historical hindsight. Even in a utopian situation, even in the present or near future here on Earth, even where mental or "psi" powers are involved, a kind of frontier ethic...